This invention relates to a gas chromatography analysis instrument, and in particular, to a trap system for preconcentration and injection of a sample which employs an adsorbent coated trap column.
Gas chromatography is unsurpassed in its selectivity, sensitivity, and cost effectiveness. It is applicable for at least several hundred thousand compounds of low to moderate boiling point including compounds in the C.sub.1 to C.sub.5 range. The process is also unique in its ability to obtain complete speciation of complex mixtures of compounds.
In gas chromatography analysis the analyte mixture is separated into its components by eluting them from a column having a sorbent by means of a moving gas. In gas-liquid chromatography, which is the type most widely used at present, the column comprises a nonvolatile liquid sorbent coated as a thin layer on an inner support structure, generally the inside surface of a capillary tube. The moving gas phase, called the carrier gas, flows through the chromatography column. The analyte partitions itself between the moving gas phase and the sorbent and moves through the column at a rate dependent upon the partition coefficients or solubilities of the analyte components. The analyte is introduced at the entrance end of the column within the moving carrier gas stream. The components making up the analyte become separated along the column and elute at intervals characteristic of the properties of the analyte components. A detector, for example, a thermal conductivity detector or a flame ionization detector (FID) at the exit end of the analytical column responds to the presence of the analyte components. Upon combustion of the eluted components at the FID, charged species are formed in the flame. The flame characteristics are monitored through a biased ion detector which, along with associated signal processing equipment, produces a chromatogram which is a time versus detector signal output curve. The trace for complex mixtures includes numerous peaks of varying intensity. Since individual constituents of the analyte produce peaks at characteristic times and whose magnitude is a function of their concentration, much information is gained through an evaluation of a chromatogram.
Gas chromatography is often used for measuring the presence of minute concentrations of organic compounds. In order to permit an evaluation of a sample it generally must be enriched and introduced into the analytical column as a narrow sample "plug" so that the components eluting from the analytical column provide an adequate signal level with good output resolution.
Great strides have been made in the development of gas chromatography systems. The technique, however, provides additional opportunities for exploitation. One of the challenges posed to designers and users of gas chromatography equipment is a desire to provide fast analysis time needed, for example, to permit industrial process stream control or monitoring of internal combustion engine exhaust gases. In such applications, time dependent changes in concentrations of the analyte components can occur. Rapid analysis time permits analysis despite such transients.
In order to enrich organic compounds in vapor streams and inject them as a narrow sample plug to the separation column, electrically heated cold traps with bare metal tubes have been used extensively. The bare metal capillary tube is cooled to cryogenic temperatures and a sample analyte is admitted. The analyte materials undergo a phase change and adhere to the inside surface of the tube. After sample collection and enrichment, the metal tube is heated by passing an electric current through it. In a very short time, the tube is heated to a temperature sufficient to vaporize the trapped compounds. This results in a concentrated vapor plug which is injected into a gas chromatograph column. Such cold trapping or "cryo-focusing" can result in large concentration enrichments and very narrow injection bandwidths for the analysis of samples.
In some applications of gas chromatography analysis there is a desire to detect and measure concentrations of extremely low boiling point gases such as methane. While bare metal cold traps operate in a generally satisfactory manner, many lower boiling point compounds are difficult to cryo-focus in a bare metal tube because of the very low temperatures needed. In many applications, for example an industrial settings or where mobile monitoring may be desired, the availability of cryogenic temperatures is limited, and if necessary, poses a significant cost penalty and limits operational flexibility.
In accordance with the present invention, a gas chromatography apparatus is provided which enables rapid evaluation time and further enables the trapping and reinjection of very low boiling point gases including methane, which is notoriously difficult to trap. In accordance with this invention, a trap is provided in the form of a capillary tube which is coated with a substance which shows retention by adsorption of the compounds of interest For such adsorbent coated cold traps, efficient trapping and large concentration enrichments can be obtained at temperatures well above conventional trapping temperatures of the compounds. This can greatly reduce the cooling requirements for these devices. In accordance with this invention, several approaches toward implementing such an adsorbent coated trap are disclosed, including a system in which sample collection and reinjection occurs as the carrier gas stream flows through the trap in one direction, and an embodiment in which gas flow direction is reversed between operating modes. In addition to the adsorbent coated trap functioning as a means of concentration and reinjection of the analyte, the retention of the components of the mixture through adsorption on the column can be used as a means of initial separation of the analyte components.
Additional benefits and advantages of the present invention will become apparent to those skilled in the art to which this invention relates from the subsequent description of the preferred embodiments and the appended claims, taken in conjunction with the accompanying drawings.